[AIRAVATA-2717] [GSoC] Resurrect User-Defined Airavata Workflows

Apache Airavata used to support user-defined workflows using the XBaya interface, to drag and drop application components to a workspace and define data flow and control flow dependencies among the application nodes. Airavata has evolved significantly and currently it only supports single job submissions through Orchestrator. But the current development version is being built-over Apache Helix for DAG orchestration. This provides an opportunity to resurrect workflow capabilities in Airavata. This GSoC project consists of the following sub tasks.

• Finalizing the Airavata Workflow Language.
• Modifying the Orchestrator to parse user-defined workflow and translate to equivalent Helix DAGs, execute the workflow, and monitor it at runtime.
• Developing a simple GUI to demonstrate the capabilities.